R Pope hit on something that reminded me of an area that a lot of people overlook on their tow rigs (overheating).

Do not run a flex fan on the rig especially if your towing over mountains with a heavy load.

What happens is that if the engine down shifts pulling a hill, the engine RPMs go up causing the fan blades to flatten out drawing less air, at the same time the vehicles speed is reduced further slowing the amount of air going through the radiator.

If you don't already have one, use a fixed fan on a HD fan clutch (preferably thermostatically controlled).